In MIMO system, multi-antenna transmission is divided into single user MIMO (SU-MIMO) transmission mode and multi-user MIMO (MU-MIMO) transmission mode.
As shown in FIG. 1, in the SU-MIMO transmission, a base station (BS) sends a plurality of data streams on the same time-frequency resource to the same user. In contrast, in the MU-MIMO transmission, a base station (BS) sends a plurality of data streams on the same time-frequency resource to different users, and improves the system capacity by transmitting data simultaneously to a plurality of users.
Pre-coding technology is a signal processing technology which pre-processes the transmitted symbols at the transmitting side by using channel state in order to eliminate interference, improve system capacity and the like. The pre-coding needs the base station to know transmission channel information and determine a pre-coding matrix based on the transmission channel information. For example, pre-coding technology may be a pre-coding technology based on codebook, in which the codebook is the same with respect to user equipment (UE) and base station (BS). A user notifies a base station of the pre-coding matrix used by the user by feeding back a pre-coding matrix indication (PMI) to the base station.
In MIMO transmission technology using the pre-coding technology, the PMI may be included in downlink channel feedback information which a user needs to feed back to a base station. The downlink channel feedback information is information for reflecting the downlink channel state which the user feeds back to the base station through uplink channel.
In MIMO systems of LTE, for example, in downlink channel feedback of SU-MIMO, the downlink channel feedback information which a user feeds back to a base station may includes: the PMI for indicating a pre-coding matrix; and a feedback channel quality indication (CQI) for selecting a modulation coding mode of transmission data by the base station. In LTE standards, in the situation of MU-MIMO transmission, the PMI and CQI in the downlink channel feedback information which a user feeds back to a base station is obtained by optimizing the PMI and CQI in the situation of SU-MIMO transmission. However, using the PMI obtained in the situation of SU-MIMO transmission in MU-MIMO transmission may induce influence to the selection of paired user in MU-MIMO transmission and thus influence to the communication performance.
In addition, from the perspective of base station side, after the user feeding back the downlink channel feedback information, the base station determines whether the MU-MIMO transmission or the SU-MIMO transmission is used according to the downlink channel feedback information. As for SU-MIMO transmission, the base station obtains the pre-coding matrix according to the PMI in the downlink channel feedback information, and selects a suitable modulation coding mode for the user according to the CQI in the downlink channel feedback information. As for MU-MIMO transmission, the base station needs to first select paired user(s) for the user according to the downlink channel feedback information fed back by the user, and then obtain a pre-coding matrix for the user and select a suitable modulation coding mode for the user according to the PMI and CQI included in the downlink channel feedback information fed back by individual paired users selected for this user.